High-efficiency tubular electrical steering servo

ABSTRACT

The invention relates to a steering servo for motor vehicles, with a drive unit arranged coaxially with the rack arrangement, which comprises a combination of a tubular electrical motor ( 9, 10, 12 ), with a recirculating-ball spindle drive. A particularly compact and cost-effective steering servo arrangement with few components is permitted, whereby the ball nut ( 7 ) of the recirculating-ball drive is mounted coincidentally with the rotor ( 9 ), which is arranged as an external rotor.

BACKGROUND OF THE INVENTION

The invention relates to a steering booster configuration for motorvehicles according to the preamble of claim 1.

Known servo steering systems are operated, for example hydraulicallysupported, thereby that on the steering shaft the control torquegenerated by the driver is measured and therewith via a hydraulic systema piston is driven which acts upon the transverse toothed rack of thesteering system, which is a portion of a thrust rod and thus generates aforce augmentation depending on the deflection of the steering wheel.Such electrohydraulic systems have the disadvantage that at high vehiclespeeds, where per se the least servo force augmentation is required, thegreatest hydraulic power is obtained and during standstill of the motorvehicle the force augmentation steeply decreases, where the greateststeering forces would be required.

Attempts have therefore been made to realize steering boosterconfigurations which drive the toothed rack directly with anelectromotor. Such systems save space and energy since the electromotoris only activated when needed. With electromotor-driven steering boosterconfigurations it would be possible to provide the auxiliary forcesupply at the correct point in time, for example correspondingly morestrongly when the motor vehicle is not moving.

U.S. Pat. No. 5,711,396 for example discloses an electromotor-drivenservo drive, which acts directly onto the toothed rack of the steeringsystem. In order to be able to generate sufficiently high forces, acorrespondingly robust electromotor is provided which requirescorresponding strong stepping-down in order to generate sufficientlyhigh controlling forces. In the prior cited patent the electromotor wasdisposed laterally to the toothed rack or thrust rod, which drives aball spindle gear which is disposed coaxially with the toothed rack axisand serves as a reduction gear. Of disadvantage in this configuration isthat the motor and the gear disposition are of a complicated structureand occupy a relatively large installation space. In addition, theefficiency is not particularly high. To save installation space,attempts have also been made to dispose recirculating ball gearstogether with the electromotor coaxially on the toothed rack axis, suchas has become known for example from GB 2 284 790. A further coaxialconfiguration was disclosed in EP 0 101 579. The configurations knownuntil today have the disadvantage that their structure is relativelycomplicated, in particular in the proximity of the bearing of the ballnut and of the motor rotor. Due to the limited efficiencies of theelectrically operated servo steering configuration, the applicationfield is also limited since these also demand high electric feed powerof the electromotor. The available electric power of the onboard networkof a motor vehicle is limited. In addition, the previous steeringboosters are insufficiently compact, which also limits utilization insmall motor vehicles or makes them not economical.

SUMMARY OF THE INVENTION

The task of the present invention resides in eliminating thedisadvantages of the above cited prior art. The task resides inparticular in realizing an electric steering booster for a steeringconfiguration, which is structured extremely compactly and which permitsattaining rapid response behavior with good dynamics with responsivesteering behavior for the driver, which operates at high efficiency andwhich can generate large controlling forces acting directly onto thethrust rod or toothed rack and which can be produced economically. Theconfiguration, moreover, is to be highly compact and is to have thecapacity of being disposed tubularly coaxially with respect to the axisof the thrust rod.

The task is solved according to the invention by the configurationaccording to the characteristics of claim 1. The dependent claims definefurther advantageous embodiments.

The task is solved according to the invention thereby that anelectromotor disposed coaxially to the thrust rod axis drives with itsrotor a rotation-translation converter. This comprises a ball-threadednut, which, in turn engages a threaded coupling disposed in a subregionof the thrust rod. The rotatable nut, or the rotor, are rotatablysupported with only one bearing and this is stationarily braced againstthe motor vehicle chassis, such that upon torquing of the nut it causesan axial displacement of the thrust rod, wherewith a corresponding forceacts onto the thrust rod. The nut and the thrust rod act like a gear forthe force transmission. The advantage of this configuration comprisesthat the rotor of the motor is coupled directly with the ball-threadednut of the rotation-translation converter and only one bearing isrequired.

As the rotation-translation force converter the already knownrecirculating ball spindle gears or recirculating-ball worm gears areespecially suitable. In such configurations the balls, which roll alongbetween the nut and the threaded turn of the threaded rod, for examplein an endless loop, are guided back again to their original position. Anespecially suitable configuration of a recirculating-ball worm gear isavailable for example from the company Amsler & Co AG in CH-8245Feuerthalen with the type designation “Star” and the name‘Kugelgewindetrieb’ (ball screw). With this model an especially compact,precise converter configuration with high efficiency can be attained.The utilization of a special, electronically commutating motor withbell-like rotor, moreover, permits the use of a single bearing for thebearing support of the ball-threaded nut together with the bell-likerotor. The electromotor disposed coaxially with the toothed rack orthrust rod axis is developed such that the stator with its windingcoaxially encompasses the thrust rod axis and the rotor, preferablytubularly, encompasses rotatably supported the stator thereover. Therotor carries permanent magnets of the type rare earths and is developedas a bell-form rotor, which is supported on one side together with theball-threaded nut. With such a configuration high degrees of efficiencycan be attained with compact implementation.

The force transmission ratio of the rotation-translation converter andof the motor is laid out such that high controlling forces can begenerated, preferably up to more than 9 kN with rapid reaction times,which is of particular importance for the steering safety. The motor isfrequency-controlled according to the desired requirements via anelectronic power stage, wherein the driving frequency lies preferablyabove the range of the human audibility limit. The power stage is drivenby a control unit, which processes as instantaneous value signal thetorque measured on the steering shaft, which is generated during thesteering with respect to the toothed rack, for this purpose for examplea torsion rod is disposed between the steering gearing and the steeringshaft and its deformation under the effect of force is measured. Thecontrol can with advantage be provided with a correspondingcharacteristic or transfer function in order to be able to generate thedesired control characteristics at different operating behavior. Forexample, the speed-dependent and weight-dependent control force behaviorcan be taken into consideration and be compensated. Furthermore, withadvantage the resetting of the front wheels upon releasing the steeringwheel can be supported and/or irregularities of the steering joint, forexample due to a universal joint error, can be compensated and/orrotational vibrations of the steering system can be actively damped. Theelectronically controlled motor permits, moreover, to equalize certainnegative properties of the motor, such as for example residual ripplebehavior or also fabrication tolerances, thereby that, for example, themotor power is compensatingly modulated either through fixed presettingor by measuring the effect via sensors and integration into a programmedcontrol system. The control is advantageously developed as programmablecontrol, for example as a microprocessor system, which can be definedthrough software or also through fixedly integrated, stored programs.With the motor converter configurations according to the invention goodoverall degrees of efficiency with compact structure and with highcontrolling forces can be achieved without overloading of the onboardnetwork.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be explained in further detail byexample with schematic Figures. Therein depict:

FIG. 1 schematically and in cross section a steering boosterconfiguration according to the invention,

FIG. 2 schematically and in cross section an enlarged configurationaccording to FIG. 1 of the motor converter configuration.

In the Figures is depicted schematically and in section a steeringbooster configuration according to the invention with an axiallydisplaceable toothed rack configuration 1 and a coaxially disposedelectromotor driving unit. The toothed rack configuration 1 is disposedin a motor vehicle in known manner such that it is coupled at its endsto the steerable wheels, for example across so-called steering tie rods,which, with the wheels, which are rotatably supported, can be pivotedthrough back and forth motion during steering such that the wheelsexperience a corresponding steering turning. The toothed rackconfiguration 1 comprises a first section 2, on whose rack subregion aredisposed teeth 5, which engage a pinion 6 driven via the steering shaftby the control (steering) wheel during the steering process. This firstrack subregion 2 with the teeth 5 forms together with the pinion 6 thesteering gearing 20 accommodated in a casing 20. Between the pinion 6and the control wheel during the torquing a force action is generatedwhich is acquired with measuring means and the resulting signal isevaluated and correspondingly via a control configuration or aregulation circuit configuration conducted to a motor driving devicewhich, in turn, couples the force onto the toothed rack configuration 1in order to permit in this way an easier steering process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The first rack subregion 2 is continued via an intermediate piece 3connecting the toothed rack subregion 2 with the second rack subregion4, with this rack portion 4 being developed as a ball spindle andconstituting a component of the drive. The outer end of the first rackportion 2 and of the second rack portion 4 in the installed state in themotor vehicle is operationally connected with the pivotable wheels. Onthe second rack portion or on the ball spindle is disposed a ball nut 7coaxially encompassing such, which nut is supported rotatably about therack axis with a bearing configuration 8, such that the nut 7 isarrested in the axial direction with respect to the installationposition and thus relative with respect to the chassis in the directionaxial to the rack configuration 1. When torquing the ball nut 7 thus theradial forces and the axial forces are absorbed on the shortest path andconducted into the casing 18, which encompasses the bearing and themotor. The torquing of the ball nut 7 thus causes a longitudinaldisplacement of the ball spindle 4 and thus of the toothed rackconfiguration 1. The bearing configuration 8 advantageously comprises ahighly loadable ball bearing, for example a double-row inclined ballbearing or also a four-point, rolling or deep groove roller bearing.

The electromotor is developed as an electronically commutated motor andcomprises a stator 10, which is developed as an electric winding and isstationarily disposed directly next to the ball nut 7 and coaxiallyencompasses the toothed rack configuration 1. This stator winding 10 canbe disposed, for example, on a casing portion 18 which in the form of atube encompasses the toothed rack configuration, and can simultaneouslyserve as protective tube or connection tube between steering gearingcasing 20 and the motor converter casing 18. As already described, thestator winding 10 is supplied from an electronic control or regulationunit. The rotor 9 of the electromotor is advantageously developed in theform of a tube or a bell as external rotor and supported directlytogether with the ball nut 7 via the bearing configuration 8. The rotor9 carries permanent magnets with a high energy product of the typerare-earths magnets, preferably of the type cobalt-samarium orneodymium.

Between rotor 9 and stator 10 is provided an active air gap 21 of lessthan 2 mm, the air gap should preferably be <1.5 mm in order to attainespecially good efficiencies. The air gap width tolerance is herein ≦10%compared to the average air gap width dimension in order to permitsufficiently good synchronization properties and thus sufficiently goodsteering behavior.

With respect to the end side of the second rank portion 4 the casing 18is advantageously provided with a connecting fitting 15 which forms acoaxial opening 16 encompassing the ball spindle 4, and thus permits anair entry opening to the motor region. On this connecting fitting 15 isdisposed, for example, a protective rubber bellows, which during theback and forth motion of the toothed rack configuration 1 exerts a typeof pumping action for air and thus makes possible through openings orrecesses 17 in the proximity of the bearing configuration 8, to move airin particular through the motor air gap 21. This causes the efficientcooling of the configuration with less expenditure.

The connection tube portion 19 which encompasses the intermediate piece3 of the toothed rack configuration 1, is developed advantageously onthe inner tube surface as a slide surface, along which a bracing element11 disposed on the intermediate piece 3 can be slid during the back andforth motion of the rack configuration 1. This bracing 11 effects aprevention of the tilting of the ball spindle rod portion 4, as aconsequence of the efficiency would be impaired due to a type of jammingin the ball-threaded spindle converter.

The bracing 11 is advantageously provided with slide material in theregion of the running surfaces, such as with suitable syntheticmaterials or corresponding slide bushings. But the bracing can also takeplace with a bracing element 11 a, disposed stationarily on the innertube 19, and in this case the slide surface is developed on theintermediate piece 3. Possible is also a combination of both types ofbracings.

On connecting fitting 15 connected with casing 18 is advantageouslyprovided the fastening of the steering booster configuration withrespect to the chassis, for example with a rubber ring damperconfiguration 13 and a holding shackle.

The configuration can be realized with high efficiency highly compactand economical with very few parts. Compact motor converter dimensionsare readily possible of less than 120 mm diameter d at a length 1 ofless than 140 mm corresponding to the embodiment according to theinvention. With these dimensions a spindle stroke of approximately 2×70mm, which means overall of 140 mm, is possible, with the spindlediameter being in the range of 24 mm at a threaded spindle pitch of 5 to10 mm. The axial force on the spindle herein reaches at least 9000 N ata rotation rate of the motor of up to maximally 800°/s and a torque ofthe electromotor of up to 10 Nm.

The steering booster configuration according to the invention comprisessubstantially very few structural parts such as the stator 10, the rotor9 with the magnets 12, the bearing configuration 8, the casing 18 withthe casing cover, the ball nut 7 and the ball spindle portion 4 or thetoothed rack configuration 1. Due to the high integration of thestructural parts, not only the size of the motor converter configurationas stated is very small, but rather also the overall length g of thetoothed rack configuration 1 from one end of the steering gearing casingto the other end of the rack configuration 1, is for example 650 mm.Moreover, through the small number of structural parts the cumulativetolerance is minimal, which makes possible the precise production atjustifiable costs and enhances the high efficiency of the configuration.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure and are entirely based on PCT/CH00/00595 and theSwiss priority application no. 2179/99.

What is claimed is:
 1. A steering booster configuration for pivotingwheels of a steerable motor vehicle in response to rotation of asteering wheel, the steering booster configuration comprising: a toothedrack configuration (1) which is movable to cause pivoting movement ofthe motor vehicle wheels, the rack configuration comprising threesubregions: a first rack subregion (2) with teeth (5), an intermediateregion (3), and a second rack region (4) which is formed as a ballspindle (4); a configuration with a pinion (6), which is rotatable withthe steering wheel and engages the teeth (5) of the first rack portion(2) in order to apply a steering force onto the toothed rackconfiguration; a ball nut (7) engaging the ball spindle (4), and beingrotatably supported with a bearing configuration (8) with respect to acasing (18); and an electromotor configuration (9, 10, 12) with a stator(10) disposed coaxially with the rack configuration (1) and a rotor (9),with the rotor (9) being operationally connected with the ball nut (7),characterized in that the bearing configuration (8) has a single bearing(8) for the unilateral bearing of the rotor (9), the rotor (9) and theball nut (7) together being rotatably supported by the bearing (8). 2.The steering booster configuration of claim 1, wherein the rotor (9) isformed bell-shaped.
 3. The steering booster configuration of claim 1,wherein the electromotor configuration (9, 10, 12) is formed as anelectronically commutated motor configuration.
 4. The steering boosterconfiguration of claim 1, wherein the stator (10) is disposed stationaryon a side facing the rack configuration (1), and the stator (10) isformed as a winding.
 5. The steering booster configuration of claim 1,wherein the rotor (9) is equipped with permanent magnets (12).
 6. Thesteering booster configuration of claim 5, wherein the permanent magnets(12) are rare-earth magnets selected from the group consisting ofcobalt-samarium and neodymium magnets.
 7. The steering boosterconfiguration of claim 1, wherein, between said rotor (9) and saidstator (10), an air gap of ≦2 mm is provided, and the air gap widthtolerance is ≦10% of the air gap width.
 8. The steering boosterconfiguration of claim 7, wherein the air gap width is ≦1.5 mm.
 9. Thesteering booster configuration of claim 1, wherein in the proximity ofthe intermediate region (3) slidable bracing means (11, 11 a) areprovided disposed between the intermediate region (3) and a guide tubeportion (19).
 10. The steering booster configuration of claim 1, whereinthe bracing means (11, 11 a) are provided in the proximity of contactsurfaces fitted with a slide material formed of synthetic material orslide sintered materials.
 11. The steering booster configuration ofclaim 1, wherein in the proximity of the bearing configuration (8) airpenetration openings (16, 17) are provided for the through-conduction ofcooling air through an air gap (21).
 12. The steering boosterconfiguration of claim 1, wherein the bearing configuration (8) is arolling bearing.
 13. The steering booster configuration of claim 12,wherein the bearing configuration is an inclined ball bearing.
 14. Thesteering booster configuration of claim 13, wherein the inclined ballbearing is formed in double-rows.
 15. The steering booster configurationof claim 1, wherein the ball nut (7) is a recirculating ball nut. 16.The steering booster configuration of claim 1, wherein the electromotorconfiguration (9, 10, 12) with the nut (7) and the bearing configuration(8) has a maximum diameter (d) of 120 mm and a maximum length (l) of 140mm.
 17. The steering booster configuration of claim 1, wherein measuringmeans for the torque measurement during rotation of the steering wheelare provided and these are operationally connected with a control and/orregulation appliance with the electromotor configuration (9, 10). 18.The steering booster configuration of claim 17, wherein the controland/or regulation appliance includes a program control for defining ortracking a presettable transmission characteristic.